It is known in the art to use inks, coating compositions, coatings, or layers, containing magnetic or magnetizable pigment particles, particularly also optically variable magnetic or magnetizable pigment particles, for the production of security elements, e.g. in the field of security documents. Coatings or layers comprising oriented magnetic or magnetizable pigment particles are disclosed for example in U.S. Pat. Nos. 2,570,856; 3,676,273; 3,791,864; 5,630,877 and 5,364,689. Coatings or layers comprising oriented magnetic color-shifting pigment particles, resulting in particularly appealing optical effects, useful for the protection of security documents, have been disclosed in WO 2002/090002 A2 and WO 2005/002866 A1.
Security features, e.g. for security documents, can generally be classified into “covert” security features and “overt” security features. The protection provided by “covert” security features relies on the concept that such features require specialized equipment and knowledge for detection, whereas “overt” security features rely on the concept of being detectable with the unaided human senses, e.g. such features may be visible and/or detectable via the tactile senses while still being difficult to produce and/or to copy. However, the effectiveness of overt security features depends to a great extent on their recognition as a security feature, because users, and particularly those having no prior knowledge of the security features of a document or item secured therewith, will only then actually perform a security check based on said security feature if they have actual knowledge of its existence and nature.
Magnetic or magnetizable pigment particles in printing inks or coatings allow for the production of optical effect layers (OEL), comprising a magnetically induced image, design or pattern which is obtained through the application of a corresponding magnetic field, causing a local orientation of the magnetic or magnetizable pigment particles in the not yet hardened coating, followed by hardening the coating. The result is a permanently fixed magnetically induced image, design or pattern. Materials and technologies for the orientation of magnetic or magnetizable pigment particles in coating compositions by applying external magnetic fields have been disclosed in U.S. Pat. Nos. 3,676,273; 3,791,864; EP 406,667 B1; EP 556,449 B1; EP 710,508 A1; WO 2004/007095 A2; WO 2004/007096 A2; WO 20051002866 A1; as well as in WO 2008/046702 A1 and other documents; therein the applied external magnetic field remains essentially static with respect to the OEL during the orientation step, as can be produced with external permanent magnets or energized electromagnets. In such a way, magnetically induced images, designs and patterns which are highly resistant to counterfeit can be produced. The security element in question can only be produced by someone having access to both, the magnetic or magnetizable pigment particles or the corresponding ink, and the particular technology employed to print said ink and to orient said pigment in the printed ink.
The magnetic orientation patterns obtained or obtainable with static magnetic fields can be approximately predicted from the geometry of the magnet arrangement, through a simulation of the three-dimensional magnetic field line pattern.
By applying an external magnetic field, a magnetic pigment particle is oriented such that its magnetic axis is aligned with the direction of the external magnetic field line at the pigment particle's location. A magnetizable pigment particle without an intrinsic permanent magnetic field is oriented by the external magnetic field such that the direction of its longest dimension is aligned with a magnetic field line at the pigment particle's location. Once the magnetic or magnetizable pigment particles are aligned, the coating composition is hardened, and the aligned magnetic or magnetizable pigment particles are therewith fixed in their positions and orientations.
Highly useful, dynamic and aesthetically appealing security features based on magnetically induced images, designs or patterns providing the optical illusion of movement can be obtained by a dynamic interaction of magnetic or magnetizable pigment particles in an hardened coating composition with a time-varying external magnetic field. In this process the magnetic or magnetizable pigment particle dynamically interacts with its surrounding coating medium, adopting a position and an orientation of lowest hydrodynamic resistance. Detailed description of the involved mechanism was given by J. H. E. Promislow et al. (Aggregation kinetics of paramagnetic colloidal particles, J. Chem. Phys., 1995, 102; p. 5492-5498) and by E. Climent et al. (Dynamics of self-assembled chaining in magnetorheological fluids, Langmuir, 2004, 20, p. 507-513).
In an attempt to produce coatings or layers comprising dynamically oriented magnetic or magnetizable pigment particles, methods for generating time-variable magnetic fields of sufficient intensity have been developed. Magnet assemblies and methods generating time-variable magnetic fields are described in EP 1 810 756 A2 and US 2007/0172261 A1 now U.S. Pat. No. 7,934,451. These means known in the art rely either on gears and shafts or on motors external to the spinning magnet to move or spin a permanent magnet within the body of a rotating cylinder of the printing or coating equipment.
CN 102529326 A discloses a magnetic orientation device comprising a drive device and a magnet, the drive device driving the magnet to rotate around a rotation shaft and the magnetic field produced by the rotating magnet being used for magnetically orienting magnetic or magnetizable pigment particles in magnetic ink printed on a substrate such as to form a magnetically oriented pattern with a three-dimensional appearance.
However, on certain printing machines, in particular sheet-fed and web-fed rotating printing machines, the mechanical constraints of the construction of the rotating cylinder of the printing or coating equipment do not allow the use of mechanical devices or electrical motors of the type known in the art. Therefore, the existing magnetic orienting devices and the technologies of the prior art do not provide for rotationally driving strong magnets within the constrained space available on the rotating cylinder of the printing or coating equipment, where only static magnets have been used hitherto.
Therefore, there remains a need for a modular, easily replaceable magnetic orienting device which fits into an existing rotating cylinder of the printing or coating equipment, and which is capable of generating a desired rotating magnetic field whilst being mechanically robust such as to withstand the acceleration forces on the printing machine.